A photonic crystal comprises an artificially engineered periodic dielectric array having at least one photonic bandgap, i.e., a range of frequencies in which ordinary electromagnetic wave propagation is strictly forbidden. The presence of these photonic bandgaps can be used to confine and guide electromagnetic waves for any of a variety of useful purposes. Guidance and confinement can be achieved by the judicious introduction of defect regions, i.e., missing or differently-shaped portions of the periodic array, within which the electromagnetic waves are permitted to exist and wherealong the electromagnetic waves can be confined and guided. Photonic crystals can exhibit special properties such as a so-called superprism effect in which, for certain frequencies, very small changes in the angle of incidence can cause very large changes in the angle of refraction. Proposals have even been made for negatively refracting photonic crystals, bringing about the possibility of so-called flat lenses or superlenses unfettered by diffraction limitations or alignment issues.
A two-dimensional photonic crystal typically comprises a horizontal slab of a bulk material into which a patterned array of vertical columns is formed, the vertical columns being occupied by a column material having a refractive index substantially different than that of the bulk material. The propagation of optical signals in these structures is determined by a variety of parameters including, for example, the cross-sectional shape of the columns, the cross-sectional dimension(s) of the columns, the inter-column pitch, the structural symmetry of the patterned array (e.g., square, hexagonal, etc.), the nature, shape, and size of any defect patterns in the photonic crystal, and the particular refractive index values of the bulk material and column material at the frequencies of interest.
Proposals have been made for dynamically modulating photonic crystals in various ways including, for example, applying external mechanical forces to cause small dimensional variations in the photonic crystal, and applying external control radiation to nonlinear bulk and/or column materials. In another proposal, solid dielectric rods are lowered into, and raised out of, air-filled columns to provide modulation. In still another proposal, microfluidic pumps reversibly fill the air holes with a fluid to change the refractive index of the columns and therefore modulate the properties of the photonic crystal.
Issues remain, however, with respect to a converse goal of performing microfluidic sensing using photonic crystal materials, i.e., sensing a property of a fluid occupying the columns by virtue of its impact on electromagnetic propagation through the photonic crystal. Such issues include, but are not limited to, device precision, sensitivity to environmental conditions, ease and effectiveness of calibration, and flexibility for different ranges of fluid parameters. Other issues remain as would be apparent to one skilled in the art upon reading the present disclosure.